The present invention relates to an image projection apparatus such as a projector, and specifically an image projection apparatus having a focus correcting function for its projection lens.
Some of image projection apparatuses such as liquid crystal projectors have an autofocus function of automatically adjusting the focus of images projected onto a screen.
In the projector, however, the projection lens expands and shrinks by the influence of temperature variations caused by light from a light source. Therefore, defocusing due to temperature (hereinafter referred to as “temperature defocusing”) is often caused even though the autofocus is performed.
In particular, immediately after power-on and immediately after switching of the projection mode (hereinafter referred to merely as “switching of the projected image”), the change of brightness of the projection image, that is, the change of the amount of projection light is large, so that the amounts of heat and light emitted from the light source and the amount of light entering the projection lens through a liquid crystal display element are widely changed. This rapidly changes the temperature in the projector and that of the optical system, thereby increasing the amount of the temperature defocusing.
In addition, a change of the environmental (ambient) temperature around the projector causes expansion and shrinkage of the projection lens, so that the temperature defocusing is caused.
Japanese Patent Laid-Open No. 2000-244847 has disclosed a technique to address these problems in which a temperature sensor is provided at the vicinity of the projection lens (focus lens) and the temperature defocusing is automatically corrected on the basis of the changes of the output from the temperature sensor.
Furthermore, Japanese Patent Laid-Open No. 2004-264570 has disclosed a technique in which the projection lens is forcibly heated in a state in which an exhaust outlet for cooling air provided on the chassis of the projector is closed to bring the projection lens into a heat-stable state. This prevents the temperature defocusing from being easily caused.
In the technique disclosed in Japanese Patent Laid-Open No. 2000-244847, however, since only one temperature sensor is provided at the vicinity of the projection lens, the response of the temperature sensor to the changes of the temperature of the projection lens itself due to the switching of the projected image is slow. Therefore, the temperature defocusing due to the switching of the projected image cannot be sufficiently corrected.
On the other hand, the technique disclosed in Japanese Patent Laid-Open No. 2004-264570 can rapidly bring the projection lens into a state in which the temperature defocusing is hardly caused by shortening the time until the projection lens is brought into a heat-stable state after power-on. However, this technique alone cannot prevent the temperature defocusing due to changes of the environmental temperature and the switching of the projected image.